ExpediaGroup / bull / #1399

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fix: prevent NPE when field transformer is applied to collection elements with flat field name mapping

When a CollectionPopulator transforms list elements (e.g. List<PersonSvc> -> List<Person>),
it calls transformer.transform(elem, targetClass) without a breadcrumb. In resolveEffectiveSource,
evalBreadcrumb("fieldName", null) returns just "fieldName", which could match a simple flat
field mapping (e.g. "name" -> "fullName"). This caused the lookup to incorrectly redirect to
the root source object instead of the current element, and getSourceFieldValue silently returned
null (swallowing the MissingFieldException because a transformer was defined), ultimately
passing null into the user's transformer function and triggering a NullPointerException.

Fix: guard the breadcrumb-based mapping lookup with isNotEmpty(breadcrumb) so root-source
redirection only occurs when there is an actual breadcrumb prefix in the path.

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